Hindered phenolic compounds are known in the art as antioxidants for the protection of organic substrates susceptible to deterioration resulting from oxidative aging. These phenolic compounds prevent the chain reaction of oxidative degradation by donating a hydrogen atom to the peroxy radical present. The resulting antioxidant radical is well stabilized and sterically hindered and thus prevented from acting as an initiator of further degradation.
All types of rubber, in particular rubbers formed from dienes, need to be protected from degradation resulting from exposure to oxygen and heating during finishing and storage. Presently alkylation and/or aralkylation products of phenols and cresols are being successfully used for this purpose. With the trend to lighter colored rubbers the discoloration of some of these antioxidants has become detrimental to the value of such rubbers. It is, therefore, desirable that the color and color stability of these phenolic antioxidants be improved and that their tendency to tint and/or discolor the final product be reduced.
During the synthesis of phenolic antioxidants it is known that treatment of the phenolic antioxidant reaction mixture with a phosphite will prevent discoloration of the antioxidant, however, the protection afforded by the phosphite is removed as soon as water is involved in the processing due to hydrolysis of phosphite in water. In the synthesis of phenolic antioxidants the reaction between the phenolic compound and the olefin (for example, isobutylene, diisobutylene, styrene, .alpha.-methylstyrene or dicyclopentadiene) is carried out in a one or two-step procedure to produce a multi-component reaction product possessing good antioxidant activity.
The reaction is usually conducted according to conventional alkylation procedures used in the preparation of alkylated phenols. See for example, U.S. Pat. Nos. 4,071,565 and 3,989,665.
U.S. Pat. No. 4,152,531 (which is herein incorporated by reference) discloses an improved process for preparing phenolic antioxidants with superior color properties. The improvement being obtained by carefully controlling process variables such as the temperature of the reaction, moisture level and catalyst level. However, U.S. Pat. No. 4,152,531 does not disclose or suggest that substituted hydroxylamines and/or oximes can be added to the phenolic antioxidant during its synthesis to provide improved color stability.